1. Field of the Invention.
The present invention relates in general to ceramic matrix composites, and in particular to a porous interfacial coating for fiber reinforced ceramic matrix composites and the methods and processes for fabricating such a coating on the ceramic matrix composite.
2. Related Art.
Ceramic matrix composites (CMCs) show considerable promise for primary and secondary structural applications for both commercial and military applications due to their low density and high temperature performance. For example, CMC's are useful for many applications because they possess high-temperature mechanical performance and exhibit a high strain-to-failure compared to monolithic ceramics. In order to achieve high strains-to-failure, many CMC systems require an interfacial coating between the fibrous reinforcement material and the ceramic matrix. Thus, interfacial coatings are becoming increasingly desirable for use with ceramic matrix composites (CMC's).
The interfacial coating is usually applied via chemical vapor deposition (CVD), sputtering, or evaporation, and provides a weak bond between the fiber and matrix that allows fiber "pullout" (ductile shear movement instead of brittle fracture) during tensile stress application. In other words, the interfacial coating creates a weak disbond layer between the ceramic matrix and fibers, thereby imparting desired ductile qualities to the final CMC.
Controlling the fiber-matrix interface of CMCs through appropriate fiber coatings is often critical to achieving high failure strengths and strains. Fiber coatings must be carefully chosen to produce the weak bond between the fiber and the surrounding ceramic matrix material to allow the fiber pullout. Carbon and boron nitride are presently the most popular interface coatings for CMCs. However, due to catastrophic oxidation, carbon is only effective to 800.degree. F. and boron nitride only to 1000.degree. F. To realize the full potential of CMC materials, where the ceramic fibers and ceramic matrix materials are often effective to temperatures greater than or equal to 2000.degree. F., fiber interface coatings must be identified that will remain effective even at these high temperatures.
However, current interfacial coatings have several limitations. First, making parts or fabrics that are coated via CVD, sputtering, or evaporation are limited in size to the internal capacity of the CVD furnace or sputtering/evaporation chamber. Also, coating uniformity is usually difficult to achieve due to line-of-sight masking during coating application. In addition, chemical vapor deposition is usually very expensive, with the cost to coat 1 square yard of fabric ranging from $2000 to $10,000, depending on the coating to be applied. Moreover, the gases used during chemical vapor deposition can be hazardous, such as boron trichloride and ammonia for deposition of boron nitride coatings.
Therefore, what is needed is a porous interfacial coating for fiber reinforced ceramic matrix composites. What is also needed is a ceramic matrix composite with a porous interfacial coating that does not have compatibility problems with the base matrix material. What is further needed is a method to apply such a porous interfacial coating to fibers that reinforce ceramic matrix composites.
Whatever the merits of the above mentioned systems and methods, they do not achieve the benefits of the present invention.